Today's commercial high field superconductors may be classified into two groups: NbTi (niobium-titanium) and Nb3Sn (niobium-tin). High temperature superconductors, discovered end 1986, can be manufactured on industrial scale but applications are limited due to short conductor lengths (ReBCO), insufficient mechanical strength (BCCO) and prohibitive high prices attaining 200 to 300 times that of NbTi.
Superconductivity is a physical phenomenon of many materials manifested by their loss of electrical resistance by cooling below the critical temperature. This temperature is material specific, e.g. for NbTi and Nb3Sn between 9 and 18 Kelvin (minus 264° C. and minus 255° C.), respectively. In comparison to conventional electrical conductors like copper, superconductors can carry without any losses much higher current, e.g. electrical power, typically 100 to 200 times more. Mainly for this reason they are used for the construction of high field magnets. Superconducting magnets are imperative for medical Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS) representing an important annual market.
Magnets need several km's of conductive wire, which is wound on a support structure. A magnetic field may be generated by putting an electrical current through the windings. In the case of a superconductor the current carrying capacity is limited by the critical current, which depends on magnetic field, temperature and mechanical stress. For the manufacturing of magnets the superconductor has to fulfill mayor requirements:
Availability in lengths of several km.
Winding of magnets without degradation of superconducting properties.
If possible, no heat treatment after magnet winding.
High mechanical strength for supporting high electromagnetic forces.
Engineering critical current density not less than 100 A/mm2 at the nominal magnetic field (temperature).
Cost efficient conductor manufacturing.
NbTi superconductors are so far the perfect material. However, the achievable magnetic field strength is limited to about 11 Tesla. For higher magnetic fields, up to about 24 Tesla, Nb3Sn superconductors are available. Nb3Sn is brittle and cannot be manufactured like NbTi. The today's fabrication technologies use the assembling of the constituent materials, which can be manufactured to a conductor by extrusion and wire drawing. Because of this complication Nb3Sn superconductors are more expensive, about 6 to 10 times that of NbTi. The superconducting Nb3Sn phase is formed after magnet winding by an appropriate reaction heat treatment in the range of 650° C. during about 100 hours. Obviously any heat treatment of the magnet increases cost.
Many attempts for the manufacturing of ternary chalcogenide of molybdenum (TMC) superconducting wires were carried out worldwide. For example, in U.S. Pat. No. 4,594,218 a superconducting wire of ternary chalcogenide of molybdenum is obtained by mixing a powder of ternary chalcogenide of molybdenum with a metal powder of smaller granularity and inserted in a metallic tube (e.g. molybdenum, niobium, tantalum, titanium and vanadium). The aim of an additional metallic powder is for removing voids and their elimination by sintering at high temperatures. After drawing and cold working the wire is subject to a final heat treatment at about 800° C. for at least twenty hours.
Further, in European patent specification EP 0 181 496 the initial superconducting material is a powdered superconducting phase, respectively the initial components for forming the phase, of an average grain size of less than 1 micrometer, which are inserted in a molybdenum tube and an outer steel jacket. Then the whole assembly is extruded at a temperature between 1000° C. and 1600° C. followed by wire drawing at elevated temperatures. It is emphasized that in European patent specification EP 0 181 496 the initial TMC material is powdered superconducting phase or respectively powdered initial components.
Another example, mentioned in the European patent application EP 0 171 918, uses TMC powder, or alternatively powder of its constituents, which is filled preferentially into a tantalum tube. The assembly is then processed into a wire and heat treated under pressure above 800° C.
Of particular relevance is the work of H. Yamasaki et al, (J. Appl. Phys. 72 (3), 1 Aug. 1992, page 1181, left-hand column, FIG. 1) who applied Hot Isostatic Pressing (HIP) after the manufacturing of a monofilamentary TMC wire with a molybdenum diffusion barrier and a stainless steel matrix. By this means the critical current density can be improved by a better connectivity of grains, as well as its uniformity of four wire samples with a length of 27 cm each (paragraph bridging page 1181 and 1182, FIG. 4). However, it is obvious to the person skilled in the art taking these data and calculate the upper critical field by a so-called Kramer extrapolation where B0.25 Jc0.5 is a linear function of the magnetic field (B is the magnetic field in Tesla and Jc is the critical current density in A/mm2). Extrapolating Jc to zero gives the upper critical field, which is 33.5 T at 4.2 K in the mentioned work of H. Yamasaki et al. This value is below of its bulk value of 51 T, indicating that the TMC wires after HIP treatment still behave granular (see also B. Seeber in Handbook of Superconducting Materials, D. A. Cardwell and D. S. Ginley (eds), Institute of Physics Publishing, 2003, Figure B3.3.5.2., p. 687). For this reason a TMC wire manufactured according to the process of the present invention can be well distinguished from the process mentioned by H. Yamasaki et al.
Other methods for the fabrication of TMC wires are mentioned in scientific literature, which is reviewed by B. Seeber in Handbook of Superconducting Materials, D. A. Cardwell and D. S. Ginley (eds), Institute of Physics Publishing, 2003, Table 3.3.5.1. p. 695. Common to all work so far, patents and publications, is that the critical current densities are insufficient for practical applications. Depending of the kind of magnet, for cost reasons magnet builder need a minimum engineering critical current density of 100 A/mm2 at the nominal field. Under engineering current density one understands the critical current divided by the total conductor cross section including non-superconducting matrix and stabilizer materials, as well as electric insulation. Comparing the most advanced TMC wire manufacturing techniques there is a convergence to about 80 A/mm2 at 25 Tesla (see B. Seeker in Handbook of Superconducting Materials, D. A. Cardwell and D. S. Ginley (eds), Institute of Physics Publishing, 2003, Figure B3.3.5.10. p. 696).
Three main reasons for the insufficient engineering current density are identified:
Poor intergrain connectivity due to the powder metallurgical manufacturing process of the conductor.
Granular behavior of the TMC superconductor. This means that the superconducting properties, in particular the critical current, are reduced at grain boundaries with respect to that inside grains.
Barrier materials other than molybdenum, e.g. tantalum or niobium, are unsuited because granular behavior is favored due to the diffusion of sulphur along grain boundaries at higher temperatures.
For the understanding of granular behavior, a brief discussion of the coherence length of superconductors is helpful. Common to all high field superconductors is the short coherence length. This length, which is a characteristic of the material, indicates the distance over which electrons forming a Cooper pair are correlated (M. Tinkham, Introduction to Superconductivity, McGraw Hill, 1975, p. 17). Cooper pairs may be considered as charge carriers in a superconductor. The coherence length can be calculated by the Ginzburg Landau relation (M. Tinkham, Introduction to Superconductivity, McGraw Hill, 1975, p. 129).
            B              c        ⁢                                  ⁢        2              ⁡          (      T      )        =            ϕ      0              2      ⁢                          ⁢                        πξ          ⁡                      (            T            )                          2            
Where Bc2 is the material specific upper critical field, Φ0 is a physical constant (magnetic flux quantum) and ξ is the coherence length. Note that Bc2, as well as ξ, depend on temperature. Once a magnetic field is exceeding Bc2 superconductivity breaks down. In other words, the higher the required magnetic field strength, the higher must be Bc2. Therefore, according the above mentioned relation, for increasing upper critical field the coherence length decreases.
The shorter the coherence length the local superconducting properties get sensitive to defects which have dimensions in the order of the coherence length or above. For instance grain boundaries may reduce locally the critical temperature, and also the upper critical field, and therefore the performance of the superconductor, i.e. the critical current. It is said that the superconductor behaves granular: with respect to the superconducting properties inside grains (intragrain properties) they are reduced at the grain boundary (intergrain properties).
NbTi and Nb3Sn with a coherence length of 6.1 nm and 4.0 nm, respectively, show almost no granular behavior. However this may happen in TMC superconductors (3.3 nm to 2.6 nm). In particular the critical current is limited by the intergranular properties. As an example the study of H. Yamasaki shows that in the case of the PbMo6S8—TMC superconductor small quantities of lead precipitations at grain boundaries reduce the critical current by one order of magnitude (H. Yamasaki et al., J. Appl. Phys. 70 (3), 1 Aug. 1991, Table I, page 1607). Further, the grain boundary physical properties are degraded in a TMC superconductor with niobium barrier by diffusion of sulphur along grain boundaries. The situation was studied in detail by P. Rabiller et al., J. Alloys Comp, 178 (1992), p. 447. One result of the study was that the chemical interaction between the TMC core and the niobium is less pronounced by reducing the recovery/reaction heat treatment temperature and by increasing the density (less porosity). A similar situation is expected in the case of a tantalum barrier.